Are natural hybrids fit or unfit relative to their parents?

Abstract

The process of natural hybridization may produce genotypes that establish new evolutionary lineages. However, many authors have concluded that natural hybridization is of little evolutionary importance because hybrids, in general, are unfit relative to their progenitors. Deciding between these alternative conclusions requires that fitness be measured for hybrid classes and parental species. Recent analyses have found that hybrids are not uniformly unfit, but rather are genotypic classes that possess lower, equivalent or higher levels of fitness relative to their parental taxa.